1. Field of the Invention
The present invention relates to a decolorable image forming material that can be decolored by heating or contact with a decoloring solvent.
2. Description of the Related Art
Forest conservation is an essential requirement to maintain the terrestrial environment and suppress the greenhouse effect caused by CO2. In order to promote wood resource saving and forest regeneration including tree planting, it is an important subject to efficiently use the paper resources that we presently possess. Currently, paper resources are “recycling” by recovering paper fibers from used paper through a deinking step of removing image forming materials printed on the used paper, remaking paper fibers to manufacture recycled paper with low paper quality, and using the recycled paper according to the purpose. Thus, problems of a high cost of the deinking step and possibility of new environmental pollution by waste fluid treatment are pointed out.
On the other hand, reuse of a hard copy has been put into practice through erasure of images, for example, by using an eraser for pencil images and a correcting fluid for ink images. Recently, rewritable paper as a type of special paper has been proposed in order to reuse hard copy paper sheets. Here, “reuse” in which a paper sheet is repeatedly used for the same purpose with preventing deterioration of paper quality as much as possible is different from “recycling” in which paper with deteriorated quality is used for other purposes. Now, the “reuse” can be said to be more important concept from a viewpoint of conservation of paper resources. If effective “reuse” at each “recycling” stage is performed, new waste of paper resources can be suppressed minimum.
The present inventors have paid their attention to a phenomenon caused by a system of a color former and a developer that a colored state is realized when interaction between the color former and the developer is increased and a decolored state is realized when the interaction is decreased. Thus, the inventors have proposed, as paper reuse techniques, image forming materials of a composition system comprising a color former, a developer and a decoloring agent having a property to capture the developer. The image forming materials can exhibit stably a colored state at temperatures around a room temperature and can retain a decolored state for a long term at practical temperatures by treatment with heat or a solvent. The inventors have also proposed image decoloring processes and image erasing apparatuses for the image forming materials. These image forming materials have advantages of high stability of colored and decolored states of the images, highly safety in view of materials, applicability to electrophotography toners, liquid inks, ink ribbons and writing instruments, and feasibility of large-scale decoloring treatment, which cannot be realized in any prior art.
The decolorable image forming materials, which we have proposed, have a great resource-saving effect, because they can promote reuse and recycling of paper and therefore can remarkably decrease waste paper. During further study of decolorable image forming materials, we have found that, if an image recording medium is made of a polar polymer such as paper and if a binder contained in the image forming material is made of a non-polar material having a property of easily capturing the color former by heating or contact with a solvent, even an image forming material without an decoloring agent having a property of capturing a developer can be decolored a few times by utilizing the ability of the image recording medium (paper) to capture the developer. Thus, we have also proposed an image forming material without the decoloring agent having the property of capturing the developer, and a method of decoloring the same (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2000-56477).
However, some problems have arisen during study of improving the composition system. The most difficult problem is limited contrast between the colored and decolored states in thermal decoloration. In the composition system, both the colored state and the decolored state are determined by equilibrium of interaction between the color former and the developer in the softened binder. For this reason, limitation of contrast is determined by temperature dependency of an equilibrium constant in the binder, and is determined by a manufacturing process temperature and a decoloring process temperature for the image forming material. That is, an optical density in a colored state (referred to as a colored optical density) is substantially determined dependent on the combination of selected materials. Therefore, there is a room for improving the colored optical density of the decolorable image forming material.